1. The Field of the Invention
The present invention is related to solid propellant compositions which are capable of burning at selected, and relatively constant, burn rates, including multiple burn rates. More particularly, the present invention is related to propellants which are formulated using one or more refractory oxides, such as TiO.sub.2, ZrO.sub.2, Al.sub.2 O.sub.3, and SiO.sub.2.
2. Technical Background
Solid propellants are used extensively in the aerospace industry. Solid propellants have developed as the preferred method of powering most missiles and rockets for military, commercial, and space applications. Solid rocket motor propellants have become widely accepted because of the fact that they are relatively simple to formulate and use, and they have excellent performance characteristics. Furthermore, solid propellant rocket motors are generally very simple when compared to liquid fuel rocket motors. For all of these reasons, it is found that solid rocket propellants are often preferred over other alternatives, such as liquid propellant rocket motors.
Typical solid rocket motor propellants are generally formulated having an oxidizing agent, a fuel, and a binder. At times, the binder and the fuel may be the same. In addition to the basic components set forth above, it is conventional to add various plasticizers, curing agents, cure catalysts, and other similar materials which aid in the processing and curing of the propellant. A significant body of technology has developed related solely to the processing and curing of solid propellants, and this technology is well known to those skilled in the art.
One type of propellant that is widely used incorporates ammonium perchlorate (AP) as the oxidizer. The AP oxidizer may then, for example, be incorporated into a propellant which is bound together by a hydroxy-terminated polybutadiene (HTPB) binder. Such binders are widely used and commercially available. It has been found that such propellant compositions provide ease of manufacture, relative ease of handling, good performance characteristics; and are at the same time economical and reliable. In essence it can be said that AP composite propellants have been the backbone of the solid propulsion industry for approximately the past 40 years.
One of the problems encountered in the design of rocket motors is the control of the thrust output of the rocket motor. This is particularly true when it is desired to operate the motor in two or more different operational modes. For example, it is often necessary to provide a high level of thrust in order to "boost" the motor and its attached payload from a starting position, such as during launch of a rocket or missile. Once the launch phase has been completed, it may be desirable to provide a constant output from the rocket motor over an extended "sustain" operation. This may occur, for example, after the rocket has been placed in flight and while it is traveling to its intended destination.
In certain applications, it may be desired to provide more than one boost phase or more than one sustain phase. For example, it may be desired to boost the rocket motor into flight, then sustain flight at a particular speed and altitude, and then once again boost the rocket motor to a higher altitude or faster speed. Such operation will at times be referred to herein as "biplateau" operation, referring to providing two or more substantially level burn rates during operation of the motor.
Until now, the performance of such multi-phased or biplateau operations has been extremely difficult. It has been necessary to resort to complex mechanical arrangements in the rocket motors. Alternatively, less efficient and less desirable liquid rocket motors have been used to obtain multi-phase operation.
In some cases, multiple-phase or biplateau operation has been attempted by constructing very complex propellant grains, such as grains having multiple propellants. In any case, achievement of multiple-phase operation has been complex, time consuming, and costly.
Accordingly, it would be an advancement in the art to provide propellant formulations which overcame the limitations of the art as set for above, and were capable of managed energy output. More particularly, it would be an advancement in the art to provide propellant formulations which were capable of operating at multiple stable outputs. Specifically, it would be an advancement in the art to provide propellant formulations which were "biplateau" in nature. Alternatively, it would be an advancement in the art to provide propellants which were capable of operating at a more precise and predictably controlled single plateau.
It would be a further advancement in the art to provide such propellant formulations in which the burn rate could be selected or changed during operation. Specifically, it would be a significant advancement in the art to provide such propellants which were capable of operating at more than one burn rate, depending on the pressure under which the propellant is burning. It would also be an advancement in the art to provide methods for using such propellants in the operation of a solid propellant rocket motor.
Such methods and compositions are disclosed and claimed herein.